Photoflash lamp and protective coating therefor



Nov. 10, 1964 J. w. SHAFFER 3,156,107

PHOTOFLASH LAMP AND PROTECTIVE COATING THEREFOR Filed NOV. 21, 1961 JOHN w. SHAFFER INVENTOR.

ATTORNEY United States Patent 3,156,107 PHOTGFLASH LAMP AND PROTEKITEVE (IOATTNG THEREFQR John W. Shatter, Wiiliamsport, Pin, assignor to Sylvania Electric Products inc, a corporation of Deiaware Filed Nov. 21, 1961, Ser. No. 153,875 3 Claims. {63. 67-31) This invention relates to the manufacture of photoflash lamps and more particularly to the composition of the protective coating with which the envelopes of these lamps are provided.

Most photoflash lamps are usually provided with a trans parent, organic, polymeric, protective coating on the outside wall of the lamp envelope. Some photoflash lamps are also provided with a similar coating on the inside wall of the lamp envelope. The interior coating serves primarily to decrease the thermal shock of combustion transmitted to the envelope during flash. The exterior coating serves primarily as a means for preventing disintegration of the glass lamp envelope in the event of a mechanical breakdown thereof resulting from thermal shock. At the present time, cellulose acetate is widely used as the exterior coating either with or without an undercoat of another polymeric material.

Over the years, the combustible, with which photoflash lamps are provided, has usually been filamentary aluminum, although in some instances magnesium and aluminum-magnesium alloys have also been employed. More recently, it has been found that zirconium is substantially better in many respects as the combustible than these previously used materials. For example, it has been found that a substantial increase in light output may be obtained when zirconium is used as the combustible. Al-

though the use of zirconium instead of aluminum as the combustible has many significant advantages, it has been noted that protective coatings of the conventional composition mentioned above which were adequate for aluminum-filled lamps are not adequate in some circumstances when shredded zirconium is employed as the combustible. For example, this appears to be particularly true when filamentary zirconium of relatively large cross section is employed. Filamentary zirconium having a shred cross section of from about 0.7 x 1.2 mils to about 1.8 x 4.5 mils is considered to be relatively large when compared with the cross section sizes of filamentary combustibles presently used commercially in most photoflash lamps. When combustibles of relatively large cross section are employed, the tendency of hot burning particles and agglomerates of the combustible to fly olf and impinge on the inner wall of the lamp bulb increases substantially. This is an undesirable circumstance because it contributes to the danger of penetration of the bulb and/ or explosion.

For certain photographic applications it is necessary to ,use a fiashlamp having a light duration of up to 50 or more milliseconds. This extended period of combustion can be obtained through the use of shreds having a relatively large cross section. When only such coarse shreds are used, however, the lamp does not attain the necessary degree of light output soon enough to permit synchronization of the flash with the camera shutter. lamps it is therefore necessary to include a quantity of shreds having a smaller cross section. These finer shreds ignite and burn rapidly thereby reducing the time needed for the lamp to attain a maximum rate of light output. Flashlamps that provide a relatively high light output per unit of volume, as well as flashlamps having an extended flash duration, are particularly difiicult to safely contain using only the former lamp envelope materials.

As noted above, the envelope which contains the combustible usually comprises a clear glass bulb having an In such 3,155,107 Patented Nov. 10, 1964 exterior, and in some cases an interior, organic polymeric protective coating. With the advent of zirconium metal shreds as the combustible material in flashlamps, it was found necessary to increase the thickness of both the inorganic and the exterior organic parts of the envelope. It Was also found necessary to omit the interior organic coating because it reduces considerably the light output of these lamps. The burning of organic vapors which are thermally expelled from an interior coating during flash consumes a significant portion of the available combustion supporting gas such as oxygen, which is needed for complete and efficient combustion of the zirconium fill.

As noted above, cellulose acetate is currently used quite extensively as the protective coating material for photoflash lamps. However, it has been found that substantial practical problems preclude the commercial manufacture of some types of zirconium flashlamps at a reasonable cost because of the thermal and mechanical limitations of the existing organic coating formulations. These coating malfunctions are manifested by flashlamp explosions and by rupturing of the containing envelope which, in turn, prevents these flashlamps from attaining their designed levels of light output. Applying a thicker conventional exterior coating to increase strength does not solve the problem because such materials exhibit considerable charring and even burning when such zirconiumfilled lamps are flashed. Charring, burning, or any thermal discoloration of the exterior coating reduces appreciably the light output of the lamp. Such thermal breakdowns of the exterior coatings are also accompanied by the evolution of a very disagreeable smoke and an attendant fire hazard.

For certain photographic applications, such as large scale illumination for example, it is desirable to construct lamps having a light output of from about 50,000 to 250,- 000 lumen seconds or more. The weight of zirconium fill, and consequently the light output of the lamp, is a function of the amount of oxygen the bulb will safely contain. Conventional large bulbs, such as the A-23 with a volume of about 250 ml. for example, cannot be used because of the limitations of existing organic coating compositions noted above. To make a zirconium-filled lamp of this size it became necessary to use a bulb such as the T-20 which has a heavier glass wall thickness. However, this bulb costs more than times as much as the A-23 because it is hand-made.

In addition to these large bulbs, it has also been found that certain other flashlamps using smaller bulbs, such as the T-6 /z for example, cannot be used with large cross section zirconium foil unless an organic exterior coating material far superior to those presently used is employed.

During the search for a stronger and more heatresistant exterior envelope coating, the following materials were investigated without success: acrylic copolymers of both thermosetting and themoplastic types; cellulosics and therrno setting cellulosics; silicone resins; water soluble polymers; vinyl copolymers; styrene copolymers; alkyd and polyester resins; melamine resins; urea formaldehyde resins; epoxy resins; polyurethane resins; polyisobutylene resins; butadiene copolymers; and certain nylon resins. For reasons of insufficient impact strength, tensile strength, or heat distortion temperature, or for any combination of such insufiiciencies, or for a lack of constancy of mechanical and thermal properties of such materials under certain conditions of temperature and/or humidity, all of the above mentioned materials failed to perform satisfactorily as an exterior coating for the types of zirconium lamps referred to previously.

I have found that polyarylcarbonate resin exhibits the necessary mechanical and thermal properties to successfully perform as an exterior polymeric coating to be used aware? in conjunction with such thin-walled bulbs, as previously mentioned, to contain the combustion of zirconium fill during flash. I have also found that lamp envelopes so constructed of a thin walled glass bulb and an exterior coating of polyarylcarbonate exhibit a great improvement over all other previously mentioned envelopes with regard to the constancy of its mechanical properties over extremes of temperature (100 C. to +100 C.) and humidity to 100% relative humidity) during storage and/ or use.

Polycarbonate resins are polymeric materials which incorporate the carbonate radical as an integral part of the main polymer chain. In polycarbonate synthesis a dihydroxy aromatic compound undergoes reaction with a carbonyl compound to yield long chain molecules which consist of alternate aromatic and carbonate groups. An example of such a polyarylcarbonate resin is the product of the reaction between phosgene and 2,2-bis(4-hydroxyphenyl) propane (bisphenol A) in the presence of a basic substance such a pyridine. The structure of this polymer is:

In commercial polycarbonate resins the number of repeating units, it is such that the molecular weight is from about 25,000 to 75,000.

Large zirconium-filled lamps, with a volume of 100 ml. or greater using inexpensive thin-walled glass bulbs such as the A-23 with a side wall thickness of from 15 to 30 mils and a zirconium shred cross section of from 0.7 x 1.2 to 1.8 x 4.5 mils, can only be flashed, without exhibiting previously mentioned envelope malfunctions, when polyarylcarbonate resin is employed as the exterior coating. Similarly, certain small zirconium-filled lamps, with a volume of 50 ml. or less using thin walled glass bulbs such as the T-6 /2 with a side wall thickness of from to 25 mils and a zirconium shred cross section of from 0.7 x 1.2 to 1.8 x 4.5 mils, can only be flushed, without exhibiting previously mentioned envelope malfunctions, when polyarylcarbonate resin is employed as the exterior-coating.

The polyarylcarbonate resin is dissolved in a suitable solvent with a fairly high evaporation rate. This provides a lacquer which is free flowing and adaptable to application by dipping or spraying against the exterior of the lamp bulb forming thereon a smooth, rapid drying coating of nearly uniform thickness.

By Way of a specific example of the invention, a completely assembled fiashlamp is dipped into a polyarylcarbonate resin lacquer so that the bulb is completely immersed as Well as a portion of the base in an area adjoining the base to bulb juncture. The lamp is then withdrawn from the lacquer and dried. As such a lacquer, I prefer to use a solution of the polycarbonate of bisphenol A in methylene chloride. From about 8% to about 22% on a weight basis-or preferably from about to about %-of the polycarbonate resin is used. Examples of polycarbonate resins which may be employed satisfactorily are Merlon #90, manufactured by Mobay, and Lexan #115, manufactured by General Electric.

By making possible the use of conventional large glass bulbs, such as the A-23 with a wall thickness of from 15 to mils and a cost of about 87.65/1000, this invention makes it economically feasible to manufacture and market zirconium-filled lamps with a light output of more than twice that of the largest currently manufactured flashlamps, for this same lamp volume. To make such a zirconium lamp it was previously necessary to use a heavy walled bulb such as the T-20 which has a side wall thickness of from 3060 mils at the formidable cost of about $860/ 1000.

In addition to making possible such large zirconiumfilled lamps, polyarylcarbonate exterior coatings have also permitted the use of smaller thin walled bulbs such as the T-6 /2 with zirconium shred cross sections of from 0.7 x 1.2 mils to 1.8 x 4.5 mils. These small zirconium shred filled bulbs can be used universally for cameras with focal plane shutters and for those with between-the-lens shutters, thereby simplifying the use and the marketing of small zirconium shred filled lamps.

In the accompanying drawing, FIG. 1 is an elevational View of one type of a photoflash lamp with which the protective coating described above may be employed and FIG. 2 is a fragmentary detail on an enlarged scale showing the protective coating on the outer wall of the glass bulb. The illustrated photoflash lamp comprises a sealed transparent bulb 2 of glass or the like, within which a stem 4 is disposed. A pair of lead-in wires 6 and 8 are supported by and extend through the stem 4, the outer ends of the wires 6 and 8 being connected to the base 10, with which the bulb 2 is provided, at 12 and 14 respectively, to provide the means through which electrical energy may be supplied to a lamp filament 16 which is attached to and supported by the lead-in wires at the inner ends thereof. The inner end of each of the lead-in wires 6 and 8 is provided with a gob of ignition paste 18. A combustible 20, in the form of shredded zirconium foil of the type described above, is disposed within the bulb 2, along with a filling of combustionsupporting gas. The outer wall of the bulb 2 is provided with a protective coating 22 of the type described above.

In the lamp illustrated in the accompanying drawing, an A-23 type bulb having a volume of about 250 ml. was used and it was provided with a gas filling of about oxygen and about 5% nitrogen at a fill pressure of about 60 cms. of mercury abs. The filamentary zirconium consisted of about 385 mgs. of zirconium shreds having a cross section of about 1.0 x 2.7 mils and 385 mgs. of zirconium shreds having a cross section of about 0.7 x 1.0 mils. The exterior polyarylcarbonate coating was about 7-10 mils thick. This lamp on firing gave a 2.0 to 2.5 megalumen level of light output for a duration of about 60-80 milliseconds.

In another embodiment, a T6 /2 type bulb having a volume of about 7.4 ml. was used and it was provided with a gas filling of oxygen at a pressure of about 120 cms. of mercury abs. The zirconium fill in this lamp consisted of about 18 mgs. of zirconium shreds having a cross section of about 0.9 x 1.2 mils and about 25 mgs. of zirconium shreds having a cross section of about 0.9 x 2.5 mls. The bulb was provided with an exterior polyarylcarbonate coating having a thickness of about 5-7 mils. The light output of this lamp was about 17,000 lumen seconds.

In a further embodiment, a bulb having a volume of about 50 ml. and a wall thickness of about 30 mils was used and it was provided with a gas filling of oxygen at a pressure of about 54 cm. of mercury abs. The zirconium fill in this lamp consisted of about mgs. of zirconium shreds having a cross section of about 0.9 x 1.2 mils. The bulb was provided with an exterior polyarylcarbonate coating having a thickness of about 5.7 mils. The light output of this lamp was about 40,000 lumen seconds.

What I claim is:

1. A photoflash lamp comprising: a sealed, lighbtransmitting envelope; a combustion-supporting gas filling in said envelope; a quantity of a filamentary combustible disposed in said envelope; ignition means disposed in said envelope in operative relationship with respect to said combustible; and a polyarylcarbonate coating on the exterior wall of said envelope.

2. A photoflash lamp comprising: a sealed, light-transmitting envelope; a combustion-supporting gas filling in said envelope; a quantity of filamentary zirconium disposed in said envelope; ignition means disposed in said envelope in operative relationship with respect to said 5 6 filamentary zirconium; and a polyarylcarbonate coating 2,964,797 Peilstocker et a1. Dec. 20, 1960 on the exterior wall of said envelope. 2,982,119 Anderson May 2, 1961 3. A photoflash lamp comprising: a sealed, light-trans- 3,106,544 L k t 1, De 8, 1963 mitting envelope; a combustion-supporting gas filling in said envelope; a quantity of filamentary zirconium having 5 OTHER REFERENCES a cross section of from about 0.7 x 1.2 mils to about Business Week article, It C0111 d Head a New Family,

1.8 x 4.5 mils disposed in said envelope; ignition means April 6, 1957, pages 99400; copy in Div. 60.

disposed in said envelope in operative relationship with Polycarbonates a Group of New Thermoplastic Synfil t l l respect to sald amen ary ZlrcOmum and a PO yary car thetic Materials, Angewandte Chenue 68, No. 20, pages bonate coating on the exterior wall of said envelope. 10 633%), Cctober 1956; py in Div. 60.

References Cited the file of thls Patent Polycarbonate Resin, by W. F. Christopher, Plastics UNITED STATES PATENTS Engineers (SPE) Journal, pages 31-34, vol. 14, No. 6,

2,787,559 cone et a1. A r. 2, 1957 June 1958; copy in Div. 60. 

1. A PHOTOFLASH LAMP COMPRISING: A SEALED, LIGHT-TRANSMITTING ENVELOPE; A COMBUSTION-SUPPORTING GAS FILLING IN SAID ENVELOPE; A QUANTITY OF A FILAMENTARY COMBUSTIBLE DISPOSED IN SAID ENVELOPE; IGNITION MEANS DISPOSED IN SAID ENVELOPE IN OPERATIVE RELATIONSHIP WITH RESPECT TO SAID COMBUSTIBLE; AND A POLYARYLCARBONATE COATING ON THE EXTERIOR WALL OF SAID ENVELOPE. 